es the difficulty with the calculations
of many enthusiasts who hail liquefied gas as the motive power of the
near future. For of course in liquefying the air power has been applied,
for the moment wasted, and unless we can get out of the liquid more
energy than we have applied to it, there is no economy of power in
the transaction. Now the simplest study of the conditions, with the
mechanical theory of matter in mind, makes it clear that this is
precisely what one can never hope to accomplish. Action and reaction are
equal and in opposite directions at all stages of the manipulation, and
hence, under the most ideal conditions, we must expect to waste as much
work in condensing a gas (in actual practice more) as the condensed
substance can do in expanding to the original volume. Those enthusiasts
who have thought otherwise, and who have been on the point of perfecting
an apparatus which will readily and cheaply produce liquid air after
the first portion is produced, are really but following the old
perpetual-motion-machine will-o'-the-wisp.

It does not at all follow from this, however, that the energies of
liquefied air may not be utilized with enormous advantage. It is not
always the cheapest form of power-transformer that is the best for all
purposes, as the use of the electrical storage battery shows. And so it
is quite within the possibilities that a multitude of uses may be
found for the employment of liquid air as a motive power, in which its
condensed form, its transportability or other properties will give
it precedence over steam or electricity. It has been suggested, for
example, that liquefied gas would seem to afford the motive power par
excellence for the flying-machine, once that elusive vehicle is well in
harness, since one of the greatest problems here is to reduce the weight
of the motor apparatus. In a less degree the same problem enters into
the calculations of ships, particularly ships of war; and with them also
it may come to pass that a store of liquid air (or other gas) may come
to take the place of a far heavier store of coal. It is even within the
possibilities that the explosive powers of the same liquid may take the
place of the great magazines of powder now carried on war-ships; for,
under certain conditions, the liquefied gas will expand with explosive
suddenness and violence, an "explosion" being in any case only a very
sudden expansion of a confined gas. The use of the compressed air in th